The carrier of a daisy wheel printer and hence the print element commonly referred to as the daisy wheel must be homed or positioned properly upon the initiation of power to the printer in order that the carrier and print position defined by the daisy wheel is in a known spacial position with respect to the remainder of the printer. This must be accomplished so that the electronic controls, which control the lateral movement of the carrier and print wheel along the platen, may maintain an accurate indication of the position of the print point and carrier position throughout the printing operations. The print wheel must be rotationally phased or homed to provide a coincidence between a known petal or character and the electronic controls to likewise insure that the elecronics can keep track of the appropriate rotational movement of the printer and thereby be able to accurately and reliably select characters for printing in accordance with the electrical signals processed by the control.
Typewriters which have removable and replacable print elements are capable of printing in more than one pitch and therefore the pitch must be selected. With the pitch properly selected, the escapement distance for each character or spacing command is appropriately sized for the size of the print on the end of the petals of the print wheel.
Printers which have the capability of printing selected ones of a plurality of different print styles, such as replaceable element printers, also are beneficially enhanced by having the ability to select and print using a selected force level.
Prior homing routines involved the driving of the carrier along the print line until the frame or other permanent obstruction interferred with the further movement and the detecting, over a period of time, that the carrier had failed to move or translate in response to the drive signals. The assumption at that time was made that the carrier occupied the leftmost position of the writing line and therefore was at the left frame.
An alternative technique which has been used is to use a microswitch such that the carrier supporting the print wheel will contact the microswitch and indicate to the microprocessor that the carrier has reached a known position while at the same time acting to terminate the further drive of the carrier against the microswitch. At this point, the microprocessor may then reset the appropriate registers to maintain a coincident indication of the location of the carrier as it is then moved outward from the left frame or homing position.
Homing of the print wheel may be accomplished by the use of a photodetection setup such as is disclosed in U.S. Pat. No. 3,574,326 wherein a hole in the print wheel provides an indication of a known position of the print wheel.
Electronic pitch sensing of the pitch of a particular print wheel has been detected by means of a feeler switch sensing the presence of holes in the cartridge of the print wheel/cartridge assembly as described in IBM Technical Disclosure Bulletin, Vol. 24, No. 1A, June 1981, pages 146, 147.
It is an object of the invention to photoelectrically detect and communicate the home position of the print wheel in both a lateral and rotary position together with the detection of the pitch for proper escapement and the font weight to insure the desired impact force being applied to each of the print wheels when printing occurs.